Introduction

The Pentax SMC-DA 21mm f/3.2 AL Limited is one of three so-called pancakes
lenses in the current Pentax lens lineup. As you can observe below pancake lenses
are pretty tiny beings - the DA 21mm has a length of merely
25mm. The species is not overly popular when looking at the whole
industry. Nikon used to have one but it has already been discontinued again and
none of the others offer one. Pancakes have advantages and disadvantages.
They're obviously as small and light-weight as it gets. Optically they're
contra light masters due to the low number of lens elements (and therefore surfaces)
and they tend to produce a relatively even image. On the downside they have a
comparatively slow max. aperture and the peak performance tends to be a little lower
compared to conventional lenses - the theory tells that the degree of correction
increases the more elements you add to the design ... assuming you do it right
(there're limits to this specifically in terms of production quality).
The DA 21mm f/3.2 is a dedicated APS-C lens so it can only be used on Pentax
(and Samsung) DSLRs. Its field-of-view is equivalent to 32mm (full format) so
it can be regarded as a perfect for street photography - a classic sweet
spot of the Pentax system.

The DA 21mm is a Limited lens which means that Pentax uses quality
material, a mainly manual assembly and an above average quality control (they say).
The lens is certainly a very high quality item with all parts made of metal
and tight tolerances. The tiny focus ring operates smooth and slightly
damped. As you can see below the lens extends a marginally when focusing towards
closer distances. The front element does not rotate so using a polarizer is
no problem. We received a user report that it is not possible to use a filter
and the lens hood at the same time because the filter prevents the hood from
locking at its intended position.

Typical for most Pentax lenses the DA 21mm f/3.2 has no internal AF motor and
relies on a slotted drive screw operated by the camera. Consequently AF operations
generate a moderate degree of noise. The AF speed is very fast on the K10D - finally
the focus group is about as light-weight as it gets. Thanks to a "Quick-Shift
Focus System" manual focusing is instantly possible once the AF has achieved
focus - this works very well and noiseless.